The present application relates to the field of radiographic imaging. It finds particular application with computed tomography (CT) scanners. It also relates to medical, security, and other applications where generating a two-dimensional projection image from data acquired from a helically scanned object would be useful.
CT and other radiographic imaging systems are useful to provide information, or images, of interior aspects of an object under examination. Generally, the object is exposed to radiation, and a two-dimensional image and/or three-dimensional image is formed based upon the radiation absorbed by the interior aspects of the object, or rather an amount of radiation that is able to pass through the object. Typically, highly dense aspects of the object absorb more radiation than less dense aspects, and thus an aspect having a higher density, such as a bone or mass, for example, will be apparent when surrounded by less dense aspects, such as fat tissue or muscle.
A radiation device typically comprises a detector array and a radiation source. In some scanners, such as three-dimensional imaging scanners (e.g., CT scanners), for example, the detector array and radiation source are mounted on opposing sides of a rotating gantry that forms a ring, or donut, around the object under examination. In a conventional CT scanner, the rotating gantry (including the source and/or detector array) is rotated in a circle situated within an x,y plane about an axis extending the z-dimension (e.g., an “isocenter”) during a scan of the object. The object is generally supported by a support article (e.g., a bed, conveyor belt) that runs parallel with and is in close spatial proximity to the mechanical center of rotation (e.g., the isocenter). As the rotating gantry is rotated, radiation is substantially continuously emitted from a focal spot of the radiation source. Radiation that traverses the object is detected by a detector array and is used to generate signals and/or data that are indicative of the object, or rather interior aspects of the object. From these signals and/or data, two dimensional and/or three dimensional (projection and/or rendered) images can be generated.
Several sub-categories of CT scanners have been developed over the years. One sub-category of CT scanner is commonly referred to as a “step-and-shoot” or “constant z-axis” (CZA) CT scanner if the radiation is emitted in the form of a fan beam or a “stationary cone beam” CT scanner if the radiation is emitted in the form of a cone beam. Herein this sub-category is referred to as a CZA scanner. In such scanners, the object remains at a constant z-position relative to the focal spot during the scan (e.g., the object is not translated along in the z-dimension with respect to the focal spot during the scan). To obtain multiple projections, multiple scans of the object can be performed, respective scans performed when the object is at different z-positions (e.g., different positions along the z-axis relative to the focal spot). That is, the object is placed at a first z-position, a first scan of the object is performed, the object is placed at a second z-position, a second scan of the object is performed, etc. It will be appreciated that a projection image, or tomogram, formed from the multiple projections can depict a larger portion of the object than a projection image formed from a single projection.
There are several features about CZA scanners that make them disadvantageous for some applications. Generally, to reconstruct a two-dimensional and/or a three-dimensional image of the object under examination, data from a plurality of projections are assembled. To obtain the data from multiple projections using a CZA scanner is time consuming because the object must be moved between scans. Therefore, for time-sensitive applications (e.g., high-throughput luggage security applications, medical applications where a patient is asked to hold his/her breath, etc.) CZA scanners are undesirable. Additionally, the object (e.g., a human patient) may be exposed to high levels of radiation because at respective z-positions, radiation is generally emitted for at least a one hundred eighty degree rotation about the object.
Another sub-category of CT scanners that has been developed is commonly referred to as a “constant-speed-helical” (CSH) CT scanner if the radiation is emitted in the form of a fan beam or a “helical cone beam” (HCB) CT scanner if the radiation is emitted in the form of a cone beam. In such a scanner, the object being scanned is translated in the z-dimension relative to the focal spot as the rotating gantry is rotated about the patient causing a helical, or spiral, scan of the object. Thus, multiple projections may be acquired from a single scan of the object. Data that is yielded from a helical scan may be referred to as helical data.
While CSH and HCB scanners may obtain multiple projections of an object more quickly (because a larger portion of an object can be scanned during a single scan) and may expose the object to less radiation than a CZA scanner that is performing multiple scans, producing images from a CSH and/or an HCB scanner may require more computational steps (e.g., interpolations) than would be required for producing images from a CZA scanner and/or may have a reduced image quality relative to projection images produced from a CZA scanner. This is because none of the scanning planes (defined as planes through which radiation travels between the radiation source and the detector that are perpendicular to the z-axis about which the rotating gantry rotates) are co-planar. Rather respective “projections” or “views” (defined as signals and/or data generated from radiation striking the detector array within a predetermined amount of time) depict a unique z-dimension of the object. Therefore, before the signals and/or data can be converted from projection space to image space, the data is interpolated using techniques known to those skilled in the art. For example, interpolation may comprise combining projections taken at equivalent “projection angles” (e.g., defined as the angular orientation of the focal spot in an x,y plane relative to the object) and at different “cone angles” (e.g., defined as the angular orientation in a y,z plane focal spot relative to the object). Because of the interpolation, images produced from CSH and HCB scanners may have a lower resolution and/or increased artifacts relative to images produced from CZA scanners.
To overcome some of the disadvantages of the CSH and HCB scanners, a technique taught in U.S. Pat. No. 5,802,134 to Larson et al. and commonly referred to nutated slice reconstruction (NSR) was developed. NSR is, in particular, used with data generated from HCB scanners and generally involves extracting parallel projections from views that are reconstructed into tilted slices (where a “slice” is defined as a set of projections that share a similar scanning plane). Respective slices are tilted at a constant angle with respect to the mechanical center of rotation but increase in cone angle. Thus, the slices can be said to be nutated with respect to each other.
While nutated slice reconstruction has proven useful for producing three-dimensional images, when producing two-dimensional projection images object distortions can appear. For example, straight edges in aspects of the object that are slanted with respect to the mechanical center of rotation may appear wavy. Such distortion may be undesirable because it may reduce image quality and/or interfere with threat detection in a security application, for example.